Such sensor arrangements, for example with eddy-current sensors having a measuring coil, have been used for years for a large variety of measurements, such as, for example, for measuring the distance to an object, for measuring the thickness of a coating layer on an object, for measuring the conductivity and magnetic. permeability of a target, or for examining the homogeneity, and for detecting damage in the structure of the target surface. Normally, measurements with the known sensor arrangements require an extensive knowledge of physical relations between quantity being determined, the measured value, and possible disturbance variables. This knowledge must often be converted to measuring and evaluation electronics that are specially adapted to the measuring problem. Some measurements cannot be carried out at all, since different influence variables superpose, so that no clear statement of the measurements is obtained. When measuring the thickness of a conductive layer on a likewise conductive carrier with a known sensor arrangement, measuring errors will occur, for example, despite an adaptation to the particular target material. This applies, for example, to the existence of local inhomogeneities, magnetization, conductivity, effective permeability, temperature gradients, etc. When using known sensor arrangements of the state of the art, it will be necessary to find again for each measuring problem the mathematical relations between influence variables and measured values. The mathematical relations are in part complex and extremely nonlinear, so that this procedure, if possible at all, will take an enormous amount of time.
The complexity of the mathematical relations between influence variables and measured values is to be demonstrated by the example of the noncontacting distance measurement by the eddy-current principle. The impedance of a coil (real part and imaginary part) varies upon approaching an electrically and/or magnetically conductive object. Thus, by measuring the impedance of a measuring coil, it is possible to determine the distance between the coil and the object being measured. In the object being measured, a current is induced, which counteracts the excitation of the coil. This reaction is again dependent on the electric conductivity and on the magnetic behavior of the measuring object, namely the material parameters. Same are again temperature-dependent. Furthermore, the impedance values are frequency-dependent, and nonlinearly related to the measuring distance. Reliable results of the measurement may be obtained only when all these influence variables are considered.